In recent years, rapid popularization of intelligent terminals and mobile applications has accelerated the development of mobile Internet, which brings an impact of a large amount of data to a mobile network. A situation in which a 2G (2nd Generation, 2nd generation mobile communications technology)/3G (3rd Generation, 3rd generation mobile communications technology) mobile network of an operator is overloaded occurs worldwide frequently. An overwhelming majority of current and future intelligent terminals support or will support a WLAN (Wireless Local Area Network). Therefore, before widespread deployment of LTE (Long Term Evolution), the WLAN, which is originally intended mainly for enterprise and family scenarios, has attracted great attention in the industry as a way of offloading data traffic from a mobile network for an operator. A top priority problem that many operators want to resolve is how to improve a network that uses the WLAN technology into a network form that is operable to a cellular operator or can interwork with a cellular network.
A cellular network may include LTE, a UMTS (Universal Mobile Telecommunications System), and a GSM (Global System for Mobile Communications). Interworking between a UMTS and a WLAN is used as an example for description.
In a process in which UE (User Equipment) is handed over from a WLAN to a UMTS, the UE decides by itself to perform a GPRS (General Packet Radio Service) attach; after completing the GPRS attach, the UE sends, to an SGSN (Serving GPRS Support Node), NAS (Non Access Stratum) information for establishing a PDN (Packet Data Network) connection, where a value of a request type in the NAS information is handover; after determining the request type, the SGSN selects a packet data network gateway (P-GW) for the UE and implements, according to an IP (Internet Protocol) address sent by the PGW, a handover from the WLAN to the UMTS.
In a process in which UE is handed over from a UMTS to a WLAN, for example, to an untrusted WLAN network, the UE decides by itself to perform WLAN access authentication; after the access authentication is completed, an ePDG (Evolved Packet Data Gateway) selects a PGW for the UE according to a handover request in a PDN connection establishment message sent by the UE and a PGW identity acquired when the authentication is performed on the UE, and implements, according to an IP address sent by the PGW, a handover from the UMTS to the WLAN.
When UE performs a handover between a cellular network and a WLAN, the UE needs to decide by itself to initiate a handover, for example, the UE decides by itself to perform a GPRS attach or decides by itself to perform WLAN access authentication. Because states of the cellular network and the WLAN cannot be acquired before the UE decides by itself to initiate a handover, the UE cannot determine a handover occasion according to the states, which not only reduces accuracy of a network handover, but also increases complexity of the network handover.